1. Field of the Invention
The present invention relates to the use and configuration of radio frequency transmitting and/or receiving coils in magnetic resonance imaging.
2. Description of the Prior Art
The body coil incorporated into the bore of most modern MR scanners is typically a birdcage design operated as a single quadrature element. At low to moderate magnetic field strengths, this design allows more or less homogeneous excitation and reception. Increased SNR in reception may be achieved through the use of radiofrequency (RF) coil arrays (Roemer P B, Edelstein W A, Hayes C E, Souza S P, Mueller O M. The NMR phased array. Magn Reson Med 1990; 16(2): 192-225).
Over the past decade, the use of radiofrequency coil arrays in MRI has increased dramatically. Benefits of many-element arrays include both SNR improvements (Roemer P B, Edelstein W A, Hayes C E, Souza S P, Mueller O M. The NMR phased array. Magn Reson Med 1990; 16(2): 192-225) and parallel imaging capability (Sodickson D K, Manning W J. Simultaneous acquisition of spatial harmonics (SMASH): fast imaging with radiofrequency coil arrays. Magn Reson Med 1997; 38(4): 591-603; Pruessmann K P, Weiger M, Scheidegger M B, Boesiger P. SENSE: Sensitivity encoding for fast MRI. Magn Reson Med 1999; 42(5): 952-962; Griswold M A, Jakob P M, Heidemann R M, Nittka M, Jellus V, Wang J, Kiefer B, Haase A. Generalized autocalibrating partially parallel acquisitions (GRAPPA). Magn Reson Med 2002; 47(6): 1202-10.). Since the advent of parallel imaging, which uses RF coil arrays to acquire multiple components of image data simultaneously rather than in a traditional sequential order (thereby accelerating image acquisition beyond previous limits), there has been a steady progression towards ever larger numbers of array elements, with ever denser arrangements. 32-channel MR systems compatible with 32-element arrays are now common, and experimental systems with 128 receiver channels have recently been developed, allowing exploration of 128-element arrays (Schmitt M, Potthast A, Sosnovik D E, Wiggins G, Triantafyllou C, Wald L. A 128 Channel Receive-Only Cardiac Coil for 3T. Fifteenth Scientific Meeting of the International Society for Magnetic Resonance in Medicine. Berlin, Germany, 2007: 245; Hardy C, Giaquinto R O, J. E. P, et al. 128-Channel Body MRI with a Flexible High-Density Receiver-Coil Array. Proceedings 15th Scientific. Meeting, International Society for Magnetic Resonance in Medicine, 2007: 244; Lee R F, Chang H, Stefanescu C, et al. A 128-channel Helium-3 Phased Array at 3T for Highly Accelerated Parallel Imaging in Hyperpolarized Gas MRI. Proceedings 16th Scientific Meeting, International Society for Magnetic Resonance in Medicine. Toronto, 2008.). Such large arrays are associated with various practical challenges, including increased bulk, cost, and complexity as compared with few-element designs. Patient-specific placement of heavy many-element arrays with dense cabling can be cumbersome, and the ability to accommodate multiple body types can be sacrificed. The use of multiple many-element arrays targeted to different body habitus or body region could also add significant expense to MR systems. Nevertheless, the benefits of many-element arrays for imaging speed, SNR, volumetric coverage, and, more recently, control of signal homogeneity and RF energy deposition (via parallel transmission (Katscher U, Bornert P, Leussler C, van den Brink JS. Transmit SENSE. Magn Reson Med 2003; 49(1): 144-50; Zhu Y. Parallel excitation with an array of transmit coils. Magn Reson Med 2004; 51(4): 775-84; Setsompop K, Wald L L, Alagappan V, Gagoski B, Hebrank F, Fontius U, Schmitt F, Adalsteinsson E. Parallel RF transmission with eight channels at 3 Tesla. Magn Reson Med 2006; 56(5): 1163-71.)) continue to drive research on and clinical use of many-element arrays.
Conventional wisdom and RF engineering rules of thumb have held that coil array elements must be placed as close as possible to the body surface in order to achieve optimal SNR performance. In select cases, loose-fitting encircling volume arrays are used, e.g. for brain imaging, where multiple head sizes must be accommodated, but it is generally assumed that SNR decreases as array radius increases and “filling factor” decreases. Many-element arrays designed for imaging of large body regions have generally been split into top and bottom halves, with each half designed to contour closely to the body surface in order to maximize SNR. Flexible many-element designs present challenges for accurate coil sensitivity calibration, since coil deformation can perturb dramatically the sensitivity of small elements. Rigid contoured designs, on the other hand, tend to accommodate only a small range of possible body types (and split designs may still suffer from some degree of motion-related sensitivity shifts). All surrounding many-element designs, furthermore, tend to fill large quantities of valuable space within the scanner bore, thereby limiting the size of subjects who may be scanned.
The SNR benefits of close-fitting coils have been assumed to be particularly applicable for parallel imaging, which relies on distinctness of coil sensitivity profiles for spatial encoding. It is known that a coil's sensitivity is generally attenuated and broadened with increasing distance from the coil, and broad sensitivities are known to result in significant noise amplification in parallel imaging. Given expectations that baseline SNR would drop and that parallel imaging capability would be reduced for coil arrays at a substantial distance from the body, there has been little impetus for exploring remote body arrays lining or incorporated into the scanner bore, and no such arrays have been incorporated into commercial products or reported in the literature.
However, detailed examination of the underlying electrodynamics indicates that remote body arrays with appropriate encircling designs actually have unexpected benefits. In particular, simulations of sample remote body array configurations suggest that, for deep tissue regions, signal-to-noise ratio (SNR) may generally be preserved, or in select situations even slightly increased, as compared with body-lining surface arrays, even in the presence of parallel imaging. New designs with further improved SNR are also possible if appropriate current paths are included in remote body arrays.